OFDM modulation is a method of encoding digital data on multiple carrier frequencies. OFDM is essentially identical to coded OFDM (COFDM) and discrete multi-tone modulation (DMT). OFDM is a FDM in which all the carrier signals are orthogonal to each other. In OFDM, a large number of closely-spaced orthogonal sub-carriers are used for carrying data. The data is divided into several parallel data streams or channels, one for each sub-carrier. Each sub-carrier is modulated with a conventional modulation scheme at a low symbol rate. OFDM is a scheme for wideband digital communication that can be used in applications namely digital television and audio broadcasting, DSL broadband internet access, wireless networks, and 4G mobile communications. In other words, OFDM is a modulation technique where multiple low data rate carriers are combined by a transmitter to form a composite high data rate transmission.
In asynchronous systems (when packet can be received at any time i.e. no time synchronized beacon or super frame) the receiver has to run all the time. This can cause a lot of power to be wasted as most of the time is spent waiting for a packet. By using a synchronizer peak to wake up the CPU in a power optimized hardware configuration, most of the system can be put in sleep mode. The CPU qualifies the peak to find the sample leading to the highest correlation score.
OFDM has gained a lot of attention for power line communication, particularly in the sub 500 KHz frequency band. There are emerging standards in this frequency band, with applications in automatic meter reading, advanced metering infrastructure, solar, and electric vehicle. PRIME, G3, IEEE 1901.2 and ITU G.hnem are some of the prominent OFDM based standards that use different flavors of OFDM. All of these standards are packetized OFDM system (vs. streaming OFDM) so they all start with a preamble signal.
OFDM synchronizer is used for wired and wireless communication for high data-rate transmission. There are 2 classes of OFDM solutions: One that uses a 256-sample size preamble and FFT (IEEE 1901.2, G3 and G.hnem) and the other that uses a 512 sample size preamble and FFT (PRIME).
OFDM is very sensitive to synchronization and since the power line channel is an extremely harsh environment, it can suffer deep and ongoing fading, distortion and noise in significant portions of the frequency band. This means that traditional preamble signals that use significant portions (or all) of the frequency band can become undetectable (this is the case with full band chirp signal used in PRIME, G3 and IEEE 1901.2). Therefore, power line communication needs a robust and flexible synchronizer particularly where communication is expected in negative SNR values.
In order to save on hardware and allow maximum flexibility with best performance, a universal synchronizer is designed that is used for all the standards namely PRIME, G3, IEEE 1901.2 and ITU G and any new flavors that are not covered by these aforementioned standards.